Movement of liquids is a key function of most technical systems, many medical devices and numerous consumer goods. Traditionally, liquids can be moved either continuously, or in a pulsating flow. Most technical liquids (e.g. water, gasoline) are very tolerant to the resulting mechanical stress when accelerating the liquid and transporting it, usually through a tubular system and/or valves.
Some liquids, however, are sensitive to shear stress, i.e. the fact that parts of the liquid move faster than other, adjacent parts of the liquid, or an adjacent mechanically rigid wall. The liquids or liquid systems of this class, therefore, are difficult to pump and special care is needed to avoid damage to the liquid. Important examples of such sensitive liquids are, ordered according to fields:                Culture liquids in biotechnological reactors for cultivation of microorganism or cells. Such dispersions need movement to facilitate transport of metabolites, injection of oxygen and removal of carbon dioxide.        Liquid systems containing fillers, e.g. as found in lacquers or coatings, where particulate material offer part of a design benefit. Typical examples are metal-micro-platelet containing surface coating for car bodies, or dispersions containing a drug of low solubility.        Food, particularly milk and other diary products.        Blood.        
In addition to applying some amount of shear stress, any pump requires an energy supply that provides the necessary energy to apply a force onto the liquid. Hence, in order to move a liquid, a pump needs to convert a source of primary energy input into mechanical movement. During pumping, the pump must in one way or the other apply a force onto the liquid. This transfer induces shear stress, thus making the design of a pump for shear-sensitive liquids challenging.
There are a broad number of technically realized designs for pumps. The large majority of currently used pumps are based on solid components, such as cylinders, pistons, blades and other rotary parts such as propellers or impellers. The use of hard materials, however, inherently creates higher shear gradients as shear is the ratio of change in velocity gradient over distance, since a hard material does not yield (deform). In contrast to this, a soft material mechanically adapts to mechanical stress
This can best intuitively be understood when comparing two examples: An iron nail is applied against the palm of a human hand. The nail does not deform, and the pressure applied results in a deformation of the soft biological tissue. In contrast, if a soft rubber stick is pressed against the palm of a human hand, both the soft rubber stick and the human hand are adapting to the force, and both deform. The same happens when a liquid is moved: A ship with a brass propeller and a diesel engine conveys force on the water of a lake by successively displacing water through pushing the hard blades of the propeller through the water. The water, in return, undergoes heavy sheer, often undergoes cavitation (formation of bubbles), and the water stream going off the propeller area is usually highly turbulent, and contains a lot of air bubbles. In contrast, a seal swimming through water has soft paws that deform during swimming, i.e. when the seal applies a force onto the water and moves its body forward. The water behind a swimming seal undergoes no cavitation, and is characterized by well structured vortices, that contain little or no air bubbles.
A specific type of pumps is known as peristaltic pump. In a peristaltic pump the fluid is contained within a flexible tube fitted inside a circular pump casing (though linear peristaltic pumps have been made). A rotor with a number of “rollers”, “shoes”, “wipers”, or “lobes” attached to the external circumference of the rotor compresses the flexible tube. As the rotor turns, the part of the tube under compression is pinched closed (or “occludes”) thus forcing the fluid to be pumped to move through the tube. Additionally, as the tube opens to its natural state after the passing of the cam (“restitution” or “resilience”) fluid flow is induced to the pump. This process is called peristalsis and is used in many biological systems such as the gastrointestinal tract.
The second key issue when designing efficient pumps is how to convert primary energy into movement. This issue is particularly pressing when pump and primary energy for driving the pump must be transported (i.e. in portable systems, or on-board systems). A particularly challenging pumping application are human heart assist devices (technical devices that help the heart of patients to move the blood through the body) or artificial heart implants such as total implants or artificial hearts, which are herein understood as a technical device pumping a patient's blood through the body, whether placed inside the body as implant or as part of life support system attached via fluid transporting tubes to a patient's circulation system. In many of these challenging applications, the available space in the thorax of a patient, or (less preferred) in the lower intestinal space, is extremely limited and a pump has to be of small volume and simultaneously very efficient to be considered as suitable.
The first generation of pumps used to replace human hearts (e.g. HeartMate I) were pulsatile devices based on a two-chamber design, where the chambers were separated by a membrane. One chamber is used for pumping the blood, while the other is filled with a gas. The gas was used to deform the membrane by changing its pressure, and thus change the volume in the blood chamber, which results in movement of the blood. This approach is similar to the biological design of the human heart since later also contains two chambers, but clinical studies showed that the design was insufficient, as patients suffered from infection (28%) and 35% of all cases underwent a failure of the device. Anticoagulants had to be applied at all times, and result in a significant number of strokes as a side-effect of the blood thinner. This design was so big, that most of it was used extracorporeal (i.e. the patient had the pump above the chest, and needed to carry a larger pump and battery pack.
A second and third generation of heart assist and replacement devices (so called mechanical circulatory support (MCS) devices) is based on the use of continuous flow, (cf) pumps, e.g. as described in N Engl J Med 2009 361 2241-51 and had a much better clinical outcome. The performance of a technical device is usually compared to the outcome of a heart transplantation which results in a survival of 84, or 80% of the patients after 1 or 2 years, respectively. The current miniaturization of the cf pumps results in higher and higher blade speed, and associated high shear stress. This is clinically evident through the appearance of pump thrombosis that is responsible for 54% of all cases where patients died as described in J Thorac Cardiovasc Surg, 2013, Volume 146, pages 437-441. This study gives recommendations, e.g. to reduce shear force, and if somehow possible, to avoid the use of a so called drive line. Later is an energy feed line (usually an electrical cable) that must go through the patients skin. Particularly this area is often responsible for infections, often with lethal outcome for the patient. Examples of such cf pumps are available from the company heartware. Arrow International (LionHeart) and Abiomed (Abiocor) circumvent this problem by using transcutaneous induction-based energy transfer, but suffered from low patient survival rate, and devices are very large.
WO 02/093665 describes a fuel cell where carbon dioxide is produced as a waste product and used for operating a pump. The carbon dioxide is used to cyclically move an actuator, which in turn cyclically increases and decreases the volume of a pumping chamber.
GB 2010385 relates to a pump where pulsating exhaust gases from a combustion engine are fed to an actuation chamber, where the cyclically move a membrane separating the actuation chamber from a pump chamber.
In the view of the above problems it is seen as an object of the invention to provide a simple pump and pumping methods, particularly for shear-sensitive liquids. In particular it is desirable to provide a pump with fully biocompatible surface, capable of avoiding thrombosis, and to pump blood in a physiologically acceptable shear-rate, so that no anticoagulant needed, hence reducing the changes for stroke, and avoid bleeding.